Microheating elements,more particularly for cathodes of electron tubes



Feb. 10, 1970 w. F. KNIPPENBERG E L 1 3,

MICROHEATING ELEMENTS, MORE PARTICULARLY FUR CATHODES OF ELECTRON TUBES Filed Feb. 20, 1968 INVENTORS WILHELMUS E KNIPPENBERG PIETER ZALM BY M AEmr United States Patent 3,495,120 MICROHEATING ELEMENTS, MORE PARTIC- ULARLY FOR CATHODES OF ELECTRON TUBES 1 Wilhelmus Franciscus Knippenberg, and Pieter Zalm, Em-

masingel, Eindhoven, Netherlands, assignors, by mesne assignments,'to U.S. Philips Corporation, New York, N.Y., a corporation of Delaware Filed Feb. 20, 1968, Ser. No. 706,897 Claims priority, application Netherlands, Mar. 7, 1967, 6703548 Int. Cl. H01j 1/94 US. Cl. 313270 2 Claims ABSTRACT OF THE DISCLOSURE A microheating element for an electron discharge tube is disclosed as a support of monocrystalline silicon carbide material and a layer of emissive material covering a portion of the surface of the support, whereby said support constitutes the heating element for the cathode.

This invention relates to microheating elements, which are to be understood in this specification to mean elements which are not longer than a few millimetres.

Such heating elements may especially be used for heating cathodes for small size electron tubes such as used, for example, in mobilophones and transistorised television sets.

Heating elements of metal have the disadvantage that their resistivity is comparatively low so that stable elements of a sufliciently high resistance cannot be obtained or with difiiculty only.

Semiconductor materials have a higher resistivity. In view of the purpose above referred to, the materials must be able to withstand high temperatures and must also be chemically resistant. This desired combination of properties is found with silicon carbide.

Such micro heating elements could be obtained by grinding olf siliconcarbide crystals, but these are comparatively expensive and diflicult to grind to size. If use is made of considerably cheaper siliconcarbide plates obtained by sintering, then difliculties are encountered in manufacturing large quantities of elements of uniform resistivity. Furthermore, sintered elements of small dimensions are very weak in mechanical respect.

The invention relates to a microheating element, especially for cathodes of electron tubes, and is characterized in that the element comprises a filamentary siliconcarbide crystal.

Filamentary crystals, which are frequently referred to in engineering as whiskers, are to be understood in this specification to mean not only crystals of circular crosssection but also crystals of polygonal cross-section and strip-shaped crystals.

Such silicon crystals may, for example, be reproducibly deposited in the required small thickness on the Wall of a space bounded by silicon carbide by recrystallisation and/ or condensation in an inert gas containing lanthanum at temperatures above 2000 C., more particularly between 2200" C. and 2600 C.

In another method the whisker crystals are grown on Patented Feb. 10, 1970 a substrate from a gas phase containing silicon and carbon by locally providing finely-divided iron on the substrate and heating to a temperature above 1200 C., during which process silicon and carbon are absorbed from the gas phase by the iron and silicon carbide crystals are deposited on the substrate.

As is well-known for heating elements, it is important for them to have a positive temperature coeflicient of the resistance. This may in the present example be achieved by carrying out the whisker crystallisation in an atmosphere containing an addition, such as nitrogen.

Whiskers of the desired thickness and resistivity can be obtained in the manner described, whereas division of the whiskers in the direction of length provides a large number of heating elements.

In order that the invention may be readily carried into effect it will now be described in detail, by way of ex ample, with reference to the accompanying diagrammatic drawing, showing a cathode having a heating wire according to the invention.

The siliconcarbide crystal is indicated by 1. Current supply wires 2 may be provided in a simple manner by connecting the ends of a whisker part 1 to a wire of a refactory metal, for example, by means of soldering materal 3 consisting of a gold alloy containing 5% by weight of tantalum. Such joints can withstand temperatures up to 1300 C. Suitable soldering materials for higher temperatures may be, for example, nickel containing 5% by Weight of molybdenum (up to 1500 C.) or tungsten (up to 1800" C.).

When using the heating element 1 in a cathode of an electron tube, it is important that the heating circuit should be separated from the emission circuit. In the heating elements according to the invention this may be achieved in a simple manner by providing an insulating layer 4 which can withstand high temperatures, for example, a layer consisting of oxide or nitride of aluminium or silicon, and applying to this layer a metal layer and a mass 5 of, for example, barium oxide containing an addition of calcium, which emits electrons upon heating.

What is claimed is:

1. A cathode for an electron discharge tube comprising, a support consisting of monocrystalline silicon carbide, a layer of emissive material covering a portion of the surface of said support, and means for connecting said support to a source of electrical current, whereby said support constitutes a heating element for said cathode 2. A heating element as claimed in claim 1, characterized in that the silicon carbide contains an addition, such as nitrogen, so that the temperature coefiicient of the resistance becomes positive.

References Cited UNITED STATES PATENTS 1,814,759 7/1931 McCullough 3l3337 3,380,936 4/1968 Masuyama et a1 252516 JOHN W. HUCKERT, Primary Examiner R. F. POLISSACK, Assistant Examiner US. Cl. X.R. 3l3337 

